Visna is a slow infection of sheep primarily affecting the lungs and nervous system. We propose to continue our investigations of the molecular basis for the slowness of infection, and for the tissue lesions. We will examine two hypotheses, gene dosage and integration, that have been advanced as explanations for slowness, using in situ hybridization and other methods for this purpose in a new model of visna. We also will map gene functions on the viral genome and use probes to regions of known function, to try to correlate viral gene products with demyelination.